1. Field of Invention
This invention relates to dynamically controlled, digitally-phased, multiple antenna elements for generating a dynamically enhanced electromagnetic field for orientation-independent tag detection and digital synthesis techniques which improves signal sensitivity of electronic article surveillance (EAS) systems.
2. Description of Related Art
An electronic article surveillance (EAS) system typically consists of (a) tags, (b) interrogation antenna(s), and (c) interrogation electronics, each playing a specific role in the overall system performance.
An EAS loop antenna pedestal(s) is typically installed near the exit of a retail store and would alarm upon the unauthorized removal of an article from the store, based on the detection of a resonating tag secured to the article. The system comprises a transmitter unit for generating an electromagnetic field adjacent to the pedestal, and a receiver unit for detecting the signal caused by the presence of the resonating tag in the interrogating field.
Some desired features in EAS include: no blind spot or null region exists in the detection zone; the interrogating field be sufficiently strong near the antenna for detecting the presence of a resonating tag in noisy environment, but sufficiently weak far away for regulatory compliance, and; the detection performance be unaffected by the orientation of the resonating tag.
One approach to suppress far field emission is to mechanically twist an O-loop antenna 180° in the middle to form an 8-loop. However, a detection null is created in the area near the intersection of the figure eight crossover due to the magnetic field lines running in parallel to the plane of the tag. This causes significantly reduced detection as optimal detection is achieved when the magnetic field lines run perpendicular to the plane of the tag.
Another approach, EP 0 186 483 (Curtis et al.), utilizes an antenna system that includes a first O-loop antenna and a second 8-loop antenna which is coplanar to the first. In such an arrangement, a circular-polarized, interrogating field is created when both antennas are driven concurrently with a phase shift such that the energy received by the tag is the same regardless of its orientation.
A different antenna structure, disclosed in EP 0 579 332 (Rebers), comprises two-loop antenna coils, wherein one coil is part of a series resonance circuit and the other coil is part of a parallel resonance circuit; the series and parallel resonance circuits are interconnected to form an analog phase-shift network which is driven by a single power source.
An equivalent analog phase-shift network is incorporated in EP 1 041 503 (Kip) that relates to a phase insensitive receiver for use in a rotary emission field.
Another approach, U.S. Pat. No. 6,166,706 (Gallagher III, et al.), generates a rotating field comprising of a magnetically coupled center loop located coplanar to an electrically driven 8-loop while overlapping a portion or both of the upper and lower 8-loops. With this antenna configuration, magnetic induction produces a 90° phase difference between the phase of the 8-loop and the phase of the center loop such that a rotary field is created.
In U.S. Pat. No. 6,836,216 (Manov, et al.), the direction of current flow in four antenna coils is separately controlled to generate a resultant magnetic field that is polarized in some preferred orientations (vertical, perpendicular, or parallel to the exit aisle) within the interrogation zone.
A plurality of antenna configurations is described in U.S. Pat. No. 6,081,238 (Alicot) whereby the antennas are phased 90° apart from each other to improve the interrogating field distribution.
EAS systems often utilize resonance effects, such as magnetoelastic resonance (e.g. acoustomagnetostrictive or AM) and electromagnetic resonance (RF coil tag). EAS tags exhibit a second-order response to an applied excitation, and the resonance behavior is mathematically described by an impulse response and a frequency response. The impulse response and frequency response from a Fourier transform may be used in two alternative means of tag interrogation: pulse-listen interrogation and swept-frequency interrogation.
EAS antennas are electrically small when compared to the wavelength at the operating frequency, typically below 10 MHz, and the interrogation zone which is within the near-field region, where the inductive coupling dominates. Planar loops are most commonly used because of its simplicity and low cost. Tag excitation requires the magnetic flux to be substantially tangential to the length of an AM tag and perpendicular to an inductive coil tag. A single antenna loop element inevitably generates an uneven interrogation zone with respect to tag position and orientation. In practice, at least two antenna elements are used to switch the field direction, thus creating a more uniform interrogation zone.
Previous solutions to the orientation problem include either simultaneously phasing or sequentially alternating multiple antenna elements.
EP 0 186 483 (Curtis, et al.) discloses an antenna structure (see FIG. 1) comprising a figure-8 loop (or 2-loop) element 11 and an O-loop (or 1-loop) element 12 that, when driven 90° out of phase, generates a constantly rotating field. Curtis's antenna structure is not well balanced, as the 0 loop generates a significantly larger field than the figure-8 loop.
EP 0 645 840 (Rebers) proposes an improved structure (see FIG. 2) that uses 2-loop element 14 and a 3-loop element 13. The 3-loop also has an advantage over the 1-loop (of FIG. 1) in terms of far-field cancellation, although it was not a concern in both Curtis's and the EP 0 645 840 (Rebers) inventions. For continuous transmission where the received signal is in the form of modulation on the carrier signal, the phase of the received signal is sensitive to tag orientation. Synchronous demodulation, or phase-sensitive detection, will not work well with a rotating field that in effect constantly rotates the tag. Quadrature receiver calculation is required to eliminate the phase-sensitivity.
EP 1 041 503 (Kip) discloses a receiver (see FIG. 3) that addresses the phase-sensitivity issue.
U.S. Pat. No. 6,081,238 (Alicot) discloses an antenna structure (see FIG. 4) that uses two adjacent coplanar single loops, where the mutual coupling introduces a phase-shift of 90°, thus creating a relatively null-free detection pattern. A practical issue with the phase-shift by means of mutual coupling is that it requires a high Q to induce 90° of phase shift between the two loops, leading to excessive ringing for pulse-listen interrogation. Also, the induced current on the coupling loop will not have as large amplitude as the current on the feeding loop, and the detection pattern will not be uniform for the two loops.
Disclosed in the same patent is a practical implementation (see FIG. 5) that alternates phase difference (either in phase or out of phase) between the two loops to switch field direction. The received signals from the two loops are shifted 90° for subsequent mixing. When the two antenna loops are in phase (during time interval A as shown in FIG. 6), there is no far-field cancellation.
Disclosed in the same patent is a solution by dividing the single loop into four equal-area elements assigned with phase of 0°, 90°, 180°, and 270°, as shown in FIG. 7.
The aforesaid methods and implementations have their specific issues and limitations. Curtis ignores the receiver and far-field cancellation. EP 0 579 332 (Rebers) uses RC phase-shifting circuit that not only introduces insertion loss but also causes resonance problems if used in a pulse-listen system. Also, RC phase-shifting circuit may not work well across a frequency range due to its limited bandwidth. For a pulse-listen system, it is simpler to sequentially alternate the 2-loop and 3-loop in terms of transmission and receiving. Alicot also uses phase-shifting circuit for quadrature receiver. As for far-field cancellation, Alicot divides the single loop into four equal-area elements. As detection performance is largely dependent upon the size of each loop element, the four-element antenna with far-field cancellation will have reduced detection compared to the two-element antenna without far-field cancellation.
All references cited herein are incorporated herein by reference in their entireties.